A wide variety of aromatic-based polymers are available for use in many industries, such as plastics manufacturing. Just a few examples of such polymers include polycarbonates, polyphenylene ethers, polyesters, copolymers such as acrylonitrile-butadiene-styrene (ABS), polyimide; and polymer blends such as those based on polycarbonate/ABS. The polycarbonates, for example, are very popular materials because of the attractive set of mechanical and physical properties they possess, such as impact resistance, heat resistance, and transparency.
Very often, aromatic-based polymer compositions require substantial improvement in their natural flame-retardant properties, e.g., the ability to resist flame ignition or the excessive production of heat and smoke during combustion. Silicone-based materials, such as siloxane fluids, have previously been used in these types of polymer compositions to improve flame retardance. For example, polyphenylene ether-phenylsiloxane copolymers have been prepared in the prior art. These materials exhibit flame ignition resistance characteristics which are much better than that seen with polyphenylene ether polymers by themselves. In similar fashion, copolymers of polycarbonate and silicones have been prepared, and these materials also exhibit enhanced flame retardance. Unfortunately, the preparation of these types of copolymers on an industrial level can sometimes be very difficult, since many large-scale plants cannot easily be modified to produce such materials.
Instead of incorporating a silicone material into the base polymer by copolymerization, the material can be introduced as an additive. For example, silicone fluids have been successfully incorporated into polycarbonates and other resins as flame retardance additives. Moreover, triphenyl silane compounds have been added to polycarbonates to provide transparent blends with good flame retardance properties, while phenylsilicone fluids have provided similar properties for polyphenylene ether compositions.
In almost all of these situations, the addition of a silicone material can adversely affect some of the properties of the base polymer. These effects arise in large part from the basic chemical incompatibility between silicones and the aromatic polymers. For example, most polycarbonate materials have a high refractive index--typically about 1.5-1.6. They are also highly aromatic and somewhat polar. In contrast, silicones usually have lower refractive index values (about 1.3 to 1.4), and possess considerable aliphatic character. They are also relatively non-polar. It is therefore not surprising that phase separation usually occurs when silicones are blended with polycarbonates--even when a very small amount (less than 0.1% by weight) of the silicone is used.
Nevertheless, the addition of silicones improves flame retardance (sometimes referred to herein as "FR"). For example, the rate at which heat and smoke are generated by the burning polycarbonate is reduced significantly when a silicone material is added to the polycarbonate. However, the parts which contain silicones are often extremely hazy or opaque.
One technique for improving the compatibility between aromatic-based polymers and silicone additives calls for the matching of the refractive indices. In general, the refractive index of silicone resins can be increased by incorporating phenyl groups along the silicone chain. This approach has enjoyed some success in the case of polycarbonate resins modified by the addition of phenyl silicone resin. However, while transparency appears to be retained, some of the specific flame retardance attributes, such as a reduction in heat- and smoke-release, do not exhibit significant improvement, and may in fact degrade.
Clearly, achieving a balance between a set of important properties in aromatic-based polymers can be a very difficult endeavor. In the case of polymers like polycarbonates, for example, flame retardance often must be achieved without sacrificing other critical properties like transparency. Moreover, other properties cannot be substantially affected in a negative fashion, such as tensile strength, impact strength, resistance to solvents and other chemicals, and the like. Furthermore, any new techniques for maximizing the various attributes of the polymer product should be compatible with current manufacturing processes for the base polymer, or should be suitable for incorporation into a base polymer resin in additive-form. These new techniques should also not raise the cost of the polymer product to an unacceptable level.